Theory of optical absorption in a simple liquid with application to liquid helium

We describe a calculation of the optical absorption spectrum of a simple liquid on the Frenkel exciton model. Moments of the imaginary part of the dielectric constant are expressed in terms of integrals involving the pair correlation function by use of the Kirkwood superposition approximation. We present numerical predictions for three of these moments in the case of liquid helium, using the experimental pair correlation function. The calculation assumes that the atoms are stationary during the absorption process. We show why a more rigorous treatment would not lead to significantly different results, compare this work with previous studies, and discuss the significance of the work and the further studies it suggests.